The present invention relates to light emitting devices and, more particularly, to semiconductor light emitting devices that include recipient luminophoric mediums.
A wide variety of light emitting devices are known in the art including, for example, incandescent light bulbs, fluorescent lights and semiconductor light emitting devices such as light emitting diodes (“LEDs”). LEDs generally include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with corresponding holes and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
Most LEDs are nearly monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by most LEDs is tightly centered about a “peak” wavelength, which is the single wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photo-detector. The “width” of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the width is measured at half the maximum illumination on each side of the emission spectrum (this width is referred to as the full-width-half-maximum or “FWHM” width).
In order to use LEDs to generate white light, LED lamps have been provided that include several LEDs that each emit a light of a different color. The different colored light emitted by the LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by surrounding a single LED with one or more luminescent materials such as phosphors that convert some of the light emitted by the LED to light of one or more other colors. The combination of the light emitted by the single-color LED that is not converted by the luminescent material(s) and the light of other colors that are emitted by the luminescent material(s) may produce a white or near-white light.
As one example, a white LED lamp may be formed by coating a gallium nitride-based blue LED with a yellow luminescent material such as a cerium-doped yttrium aluminum garnet phosphor, which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce. The blue LED produces an emission with a peak wavelength of, for example, about 460 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being down-converted, while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits yellow fluorescence with a peak wavelength of about 550 nm (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light that is emitted by the coated LED may appear white to an observer. Such light is typically perceived as being cool white in color, as it is primarily comprises light on the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red-light emitting luminescent materials such as Eu2+ doped CaAlSiN3 based phosphor particles may be added to the coating.
Phosphors are the luminescent materials that are most widely used to convert a single-color (typically blue or violet) LED into a white LED. Herein, the term “phosphor” may refer to any material that absorbs light at one wavelength and re-emits light at a different wavelength in the visible spectrum, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Thus, the term “phosphor” encompasses materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors may absorb light having first wavelengths and re-emit light having second wavelengths that are different from the first wavelengths. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths. In addition to phosphors, other luminescent materials include scintillators, day glow tapes, nanophosphors, quantum dots, and inks that glow in the visible spectrum upon illumination with (e.g., ultraviolet) light.
A medium that includes one or more luminescent materials that is positioned to receive light that is emitted by an LED or other semiconductor light emitting device is referred to herein as a “recipient luminophoric medium.” Exemplary recipient luminophoric mediums include layers having luminescent materials that are coated or sprayed directly onto a semiconductor light emitting device or on surfaces of the packaging thereof, and clear encapsulents (e.g., epoxy-based or silicone-based curable resin) that include luminescent materials that are arranged to partially or fully cover a semiconductor light emitting device. A recipient luminophoric medium may include one medium layer or the like in which one or more luminescent materials are mixed, multiple stacked layers or mediums, each of which may include one or more of the same or different luminescent materials, and/or multiple spaced apart layers or mediums, each of which may include the same or different luminescent materials.